1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic appliance are all semiconductor devices.
2. Description of the Related Art
A technology by which transistors are formed using semiconductor thin films over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor is disclosed in which a semiconductor thin film is formed using an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration lower than 1018/cm3 (for example, see Patent Document 1).
On the other hand, as well as a semiconductor thin film, a gate insulating film influences performance of a transistor. As characteristics needed for a gate insulating film, for example, low threshold voltage, high withstand voltage, excellent interface properties with a semiconductor thin film, or the like can be given. When a silicon-based semiconductor material is used for a semiconductor thin film, a silicon oxide film which has excellent interface properties with the silicon-based semiconductor material is used.
As a gate insulating film for a top-gate transistor in which a silicon-based semiconductor material is used for a semiconductor thin film, a gate insulating film in which a silicon oxide film and a silicon nitride film are stacked is disclosed (for example, see Patent Document 2).
A gate insulating film in which a silicon nitride film and a silicon oxide film are stacked can have a larger thickness than a single-layer gate insulating film of silicon oxide because a nitride silicon film having a higher dielectric constant than a silicon oxide film can be used as part of the gate insulating film. When the thickness of the gate insulating film is made large, a withstand voltage of the gate insulating film can be improved.